1. Field of the Invention
This invention relates to the field of instrumentation for nuclear reactor vessels such as pressurized water reactors. In particular, the invention provides an integrated head structure having a shroud which encloses retractable instrumentation above the reactor vessel head or cover. The head structure provides a structural support for use when the head is removed as a unit from the reactor vessel, with integral shielding for elements which have been exposed to radiation. The head structure also provides a means for protecting the rod position indicators and control rod drive mechanisms from the heat of the reactor vessel, by providing a cooling air path.
2. Prior Art
In a nuclear reactor such as a pressurized water reactor, a sealed reactor vessel houses a number of fuel rods in assemblies fixed in a vertical orientation between upper and lower core plates in the reactor vessel. Mechanisms are provided above the head of the reactor vessel for lowering control rods into spaces between the fuel rods by a required axial distance, for controlling the level of nuclear flux by absorbing a portion of the neutrons and gamma rays emitted by the fission process. Additionally, a plurality of instrumentation tubes are provided at spaces between the fuel rods in the fuel assemblies for sensing the conditions in the core, such as neutron or gamma flux levels, exit coolant temperature and the like. The control rod positioning apparatus and the instrumentation tubes for the sensors (or their connecting cables) extend through pressure penetrations in the head of the reactor vessel. When the reactor is to be serviced, the control rods are lowered into the assemblies which carry the fuel rods, and the instrumentation tubes are retracted from the fuel assemblies. The head of the reactor vessel is unbolted and lifted away using a polar crane provided in the containment structure for the reactor vessel, and the head with its depending structures is placed on a support structure or pedestal above a pool of water. The lowermost portions of the instrumentation tubes are the most heavily irradiated, and remain below the reactor head in the pool, or may be cut away and replaced.
A reactor design having instrumentation tubes extending through the head of the reactor vessel is preferred over a design having the instrumentation entering the core from below. There are a number of known variations, including for example instrumentation tubes which are guided through a top entry around a curve to engage the fuel assemblies from below. Top entry avoids forming openings or seals in the bottom of the reactor vessel, and is safer in the event of an accident. The top entry design, however, results in a rather complex structure because the instrumentation tubes must interface with a plurality of control rod positioning devices and instrumentation tube connections over the head of the reactor vessel. It also may be necessary to protect these devices and connections from the heat of the reactor vessel to ensure proper operation.
The instrumentation tubes and/or the electrical connections for the tubes are guided from one or more sealed entries through the head of the reactor vessel to instrumentation thimble tubes in a plurality of fuel assemblies in the core, by guides which are disposed below the head and form parts of the internal reactor structure. are shown in U.S. patents U.S. Pat. No. 3,827,935--Gruner et al; U.S. Pat. No. 3,853,702--Bevilacqua et al; U.S. Pat. No. 4,765,947--Babin et al; and U.S. Pat. No. 4,983,351--Tower et al. Normally, at least one sensor tube is provided for each fuel assembly in the reactor core.
In the Westinghouse AP600 design which is the subject of the invention, a supply of cooling air is provided to the portions of the control rod drive mechanisms and instrumentation in the area above the head of the reactor vessel. The control rod drive mechanisms in reactors can be shrouded so as to confine cooling air to the area needing cooling. In known arrangements, the shroud is a lightweight enclosure. When the top of the reactor is removed and stored, thus exposing the irradiated lower portions of the instrumentation tubes, it is often necessary to erect temporary shielding around the head structure, and in particular the depending instrumentation tubes, to avoid exposing workers to undue levels of radiation.
The cooling air shroud for the control rod drive mechanism in the AP600 design extends from the reactor vessel head to a seismic support plate. The cooling air shroud enables the control rod drive assembly to be raised from the reactor vessel together with the head of the reactor vessel. The support apparatus generally, and the seismic support in particular, ensure that the control rod drive mechanism remains vertically over the fuel assemblies such that the control rods are freely movable into the fuel assemblies to damp nuclear flux, and if necessary, to shut down the reactor in case of seismic disturbance or even missile attack.